Kotlin basics (vs Java)

Java vs Kotlin

Kotlin is not just Java - only Kotlin/JVM is mentioned in this blog post.

But keep in mind, there’s also Kotlin/Native, Kotlin/Wasm (webassembly) and Kotlin/JS

main function

Java: public static void main

public class Main {
	public static void main(String[] args) {
		System.out.println("Hello world");
	}
}

Java in JDK 21 preview feature: 简化了main函数写法, 可以省去explicitly声明class的步骤, 可以省去public和static

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void main(String[] args) {
	System.out.println("Hello world");
}

Kotlin

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fun main(args: Array<String>) {
	print("Hello world")
}

Class constructor

Kotlin: 自动生成all-args constructor和getter

1	`class Person(val name: String, val age: Int)`

Equivalent to Java

class Person {
	private final String name;
	private final int age;

	public Person(final String name, final int age) {
		this.name = name;
		this.age = age;
	}

	String getName() { return name; }
	int getAge() { return age; }
}

Kotlin: 如果是var而非val, 则会自动生成getter和setter

1	`class Person(var name: String, var age: Int)`

Equivalent to Java

class Person {
	private String name;
	private int age;
  
	public Person(final String name, final int age) {
		this.name = name;
		this.age = age;
	}
  
	String getName() { return name; }
	void setName(String name) { this.name = name; }
	int getAge() { return age; }
	void setAge(int age) { this.age = age; }
}

Data class

Data class除了拥有上述的自动生成constructor, getter, setter的功能外,还能生成toString(), hashcode, equals, componentN, copy方法

Kotlin

1	`data class Person(val name: String, val age: Int)`

以上kotlin代码完全等同于以下的java代码

自动生成构造函数, 且参数包含@NotNull annotation和final关键字, 代表常量
自动生成getter和setter. 由于以上使用 val, 所以只有getter没有setter
- 使用kotlin则可以直接通过p.name访问, kotlin会自动转换成getter或setter的调用
- 使用java则必须使用p.getName()才能访问
自动生成toString(), hashcode, equals
- 如果不想使用kotlin自动生成的toString, 可以手动 override fun toString(): String 代替
自动生成componentN()方法用于解构赋值

Java

class Person {
	private final String name;
	private final int age;
  
	public Person(@NotNull final String name, final int age) {
		this.name = name;
		this.age = age;
	}
	
	// getter (so setter here because fields are final)
	public String getName() { return name; }
	public int getAge() { return age; }
  
	// toString
	@Override
	public String toString() {
		return "Person(" + "name='" + name + ", age=" + age + ')';
	}
  
	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Person person = (Person) o;
		return age == person.age && Objects.equals(name, person.name)
	}
}

如果使用Java+Lombok也有类似的功能

Java + Lombok

import lombok.Data;
import lombok.NonNull;

@Data
public class Person {
	@NonNull
	private String name;
	private int age;
}

// lombok.Data will generate getters, setters, hashCode(), toString() and equals()

Kotlin

如果类的成员带上@JvmField 则不能自己定义getter和setter, 且不能设置为private

1 2	`data class Person(val name: String, @JvmField val age: Int) // If you access this class in Java, you will see: getName(), setName(), age`

支持private

1	`data class Person(private val name: String)`

支持默认值

1	`data class Person(private val name: String = "Default")`

Don’t forget Kotlin also supports 1 or more init codeblock and 1 or more secondary constructor

data class Person(val name: String, val age: Int) {
	init {
		println("Person created with name $name and age $age")
	}
	// secondary constructor
	constructor(name: String): this(name, 0)
}

Methods

Java

class Person {
	private String name;
	private int age;
	
	public void print() {
		System.out.println(name + " " + age);
	}
}

Kotlin

函数是Kotlin的一等公民, 函数能够当作普通变量存储, 或作为参数传递

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data class Person(val name: String, val age: Int) {
	fun print() = println("$name, $age")
}

Lambda函数, 如果只有一个入参, 则可以用it代表之

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val lambda: (Int) -> Boolean = { num -> num % 2 == 0 }
// can be simplified to:
val lambda: (Int) -> Boolean = { it % 2 == 0 }

Lambda函数本身是最后一个参数时, 可以移除括号

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listOf(1,2,3).filter({it % 2 == 0})
// can be simplified to:
listOf(1,2,3).filter{it % 2 == 0}

NullSatety

Kotlin has null-safety from language level.

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val p: Person = Person("a", 2)  // ok
val p: Person = null            // compile error
val p: Person? = null           // ok

Kotlin has “?” chained safety

Kotlin

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var maybeName: String? = "Bob"

var length: Int? = maybeName?.length